Purification of Iodotyronine Deiodinase. Solubilization and purification of the NADPH-dependent iodotyrosine deiodinase from bovine thyroid microsomes will be approached by methods successfully used in purifying a dithionite-dependent deiodinase (Metabolism, 19, 785 1970, and Program of the 49th Meeting of the American Thyroid Assoc., Sep 12-15, 1973, page T-19). Because of the apparent importance of -SH groups for the activity of the NADPH-dependent thyroid microsomal enzyme, protecting these groups during detergent or enzymatic solubilization of the enzyme, will be tried. Extraction microsomes, characterization of biologic electron carriers that may mediate the electron flow from NADPH to flavin will be attempted. The inhibitory effect of excess iodide upon organic binding of iodide by the thyroid will be studied by (a) observing the relationship between iodide concentration in the medium of incubated thyroid slices and the rate of hydrogen peroxide formation, and (b) comparison of the iodothyronine content of thyroids before and after administration, to rats, of blocking quantities of inorganic iodide and of thiocarbonamide antithyroid drugs. The tissue and subcellular distribution of enzyme(s) that convert 1-T4 to 1-T3 will be studied, using various bovine and rat tissue slices and homogenates; conversion will be assayed (a) isotopically (using I131-labelled 1-T4 and paper chromatographic methods) and (b) by radioimmunoassay of T3.